In the development and manufacture of modern electronic circuits, testing of the circuits, whether it be for design debug, design confirmation, or the troubleshooting of defective circuits, is an indispensable activity. When probing the electronic signals at individual locations on the circuits, it is usually necessary to hold or attach the tips of two electronic test probes to the circuit. The circuit could be located, for example, on a printed circuit board wherein the probing of a plated thru-hole via is a common choice. Vias are a popular probe location choice to probe as they provide, among other advantages, mechanical support in keeping the probe tip from slipping off the test point. Also, if, as is common, the printed circuit board is covered with a solder mask, scrapping the solder mask from the vias in order to obtain good electrical contact for the probe tips usually causes only minimal damage to the printed circuit boards.
There are two standard approaches to probing a printed circuit board via. The first approach is to manually hold the probe in contact with the outer layer via pad, and the second is to solder a probe tip to the outer layer via pad. Note that in either case, if present solder mask must be removed from the via in order for the test probe to make good electrical contact.
The manual contact approach requires two contact points, the first of which is usually driven at a signal potential while the second is usually held at ground potential while a measurement is made. In typical instances, an engineer attempts to balance a non-flexible probe tip on two very small plated thru-hole vias, each of which is approximately 0.020″ in diameter. In the common situation in which solder mask covers the outer layer metal including the vias of the printed circuit board, it is necessary to scrape the solder mask off of the via pad, as well as any oxide that may have formed on the via pad metal. It is difficult to maintain two solid contact points for any extended period of time. Also, before the measurement is taken, the probe tips are usually filed down to a sharp point because the geometries involved are so small. When performed by hand, these types of modifications to the probe tips are difficult to control and, as such, may result in sufficient damage to the probe tips that they are no longer usable.
The soldering approach also requires two points of contact for a test. Again, the solder mask and any oxide existing on the via metal must be scrapped off of the via pad prior to beginning the test. Following this action, a small amount of solder is applied at the soldering temperature to the via and to a small wire to which the probe tip can be attached, forming both an electrical and a mechanical bond to hold the wire to the contact point on the printed circuit board. The soldering of the wire to the printed circuit board is a difficult procedure due to the smallness of the board attachment point. In addition, upon removal of the probe, the via pad may be inadvertently pulled free of the printed circuit board. Should this happen, the circuit path around the via may be opened, thereby, resulting in a destructive test. Even if the probe tip is successfully removed, there is usually a significant amount of solder left on and around the via pad. This remaining solder can result in additional capacitive coupling or even shorting to adjacent conducting geometries on the printed circuit board, thereby adversely affecting the signal integrity of the system.